126 Intelligence and Miscellaneous Articles. 



by illumination, in this find an explanation. — Wiedemann's An- 

 nalen, xl. p. 647 (1890). 



ON THE EXPERIMENTS OF HERTZ. BY L. BOLTZMANN. 



I have successfully demonstrated to a large audience the sparks 

 which pass between a knob and a point in Hertz's experiments 

 on rays of electrical force, by connecting the knob with a very 

 sensitive electroscope, and the point with the pole of a suitable 

 galvanic battery, the other pole of which was put to earth. As 

 long as there were no minute sparks or scintillce (Eiinkchen*) the 

 electroscope remained uncharged ; but when these were formed 

 they established a connexion between the knob and the point and 

 caused the leaves to diverge. In this manner I was able to show 

 with certainty the scintilla produced by a single primary spark 

 when the primary and secondary inductors were at a distance of 

 36*8 metres apart, the greatest distance available. 



At a distance of 8*7 metres all the Hertz experiments could be 

 most conveniently demonstrated before an audience of about 200, so 

 as to be visible to every one ; only three or four primary sparks were 

 required for each experiment, by which the electrodes could be kept 

 brightly polished. Dust, cracks in the metal, or an impure greasy 

 condition of the surface appeared more injurious than the layer of 

 oxide : cleaning with dilute sulphuric acid, distilled water, and 

 then dry rubbing was found to be best ; a thicker layer of oxide 

 is removed by polishing with Trench chalk (moistened with spirit), 

 in which case dry rubbing is sufficient. I was not able to perceive 

 any advantage in a current of air as recommended by Dr. Classen. 



The length of the secondary spark produced at the great distance 

 of 36*8 metres I estimated at g-Q 1 ^ millim. as the maximum. As 

 the dry pile which served to charge the electroscope had a potential 

 of 200 volts, the knob and the point were adjusted at a distance 

 which exceeded by about -^q\^ the striking distance of the dry pile, 

 the discharge of which was then set up by the addition of Hertz's 

 waves to the tension of the dry pile. By using a much more 

 feebly charged battery instead of the dry pile, and a much more 

 delicate galvanoscope, Hankel's for example, the rays might be 

 perceived at still greater distances. In the latter case it might not 

 be impossible to measure the deflexion with a view to quantitative 

 determinations. The regulation of the distance between the knob 

 and the point must then be made so much the finer. 



I also made experiments on interference by reflecting the waves 

 from the primary parabolic mirror through two plane mirrors, which, 

 like those of Fresnel, formed an obtuse angle, and whose planes 

 were at a distance of fourfold a half wave-length. This method 



* It may be convenient to use the word scintilla, spark, or little spark, to 

 denote the very minute spark produced in the secondary circuit in Hertz's 

 experiments. — E. Atkinson. 



